The high strength-to-weight ratio and outstanding seawater corrosion resistance of titanium alloys have long been recognized and have marked titanium as an important structural material in future ocean systems. A specific application in which alloy titanium has several potential advantages over high-strength steels is in the construction of pressure hulls for deep submergence vehicles. Prior to utilizing alloy titanium for this application, however, definite advances must be made in fabrication technology to provide more economical procedures for joining titanium into structural components. Specific factors which control the economics of joining titanium alloys include the need for extreme cleanliness and careful gas shielding to prevent harmful contamination during fusion welding, the high cost of available weld filler materials (approximately $40 per pound), and the unavailability of an electrode for shielded-metal-arc welding (SMAW). Based on the above considerations, attention must focus on the development of new fabrication procedures and concepts for use in the construction of economical titanium pressure hulls. One such fabrication technique is diffusion welding, a process which has already been applied to the manufacture of relatively complex, small aerospace structural components of alloy titanium. Among the advantages of diffusion welding are the elimination of costly weld filler materials, elimination of the metallurigical inhomogenity of cast weld metal in the final assembly, improved mechanical properties across the joint, and elimination of much of the distortion and shrinkage problems inherent with fusion welding.
Diffusion welding is defined as the coalescence of two clean metal surfaces in the solid state by the application of controlled temperature and pressure. To achieve the surface condition necessary for diffusion welding, surface contaminants such as oxide films and absorbed gases must be removed and excessive surface roughness must be eliminated. Both contaminants and surface roughness act to limit the area of contact between the surfaces to be welded.
It has been proposed that the diffusion welding process in titanium alloys involves three interrelated stages. During the initial stage, surface oxides are eliminated and intimate contact is produced between the surfaces to be welded in order to form an interfacial grain boundary and a plane of microvoids at the interface. Oxide removal occurs as a result of the dissociation and dissolution of surface oxides at the welding temperature and is accelerated by the simultaneous application of pressure at the interface. Pressure also serves to eliminate surface irregularities and to establish the required interfacial contact. During the second stage, two changes occur simultaneously. The microvoids along the interface shrink, and most are eliminated by diffusion. Also, the interfacial grain boundary migrates out of the plane of the original interface to a lower energy equilibrium configuration. Pressure application is not required during this stage. The final stage of the welding process consists of eliminating voids by volume diffusion.
In the diffusion welding process outlined above, temperature selection is perhaps the most critical parameter since it determines the degree and rate of surface oxide dissociation and dissolution necessary to ensure surface cleanliness. In addition, temperature controls the yield strength and surface creep rate of the material, which in turn establish the amount of pressure required to achieve intimate surface contact. Finally, temperature controls the diffusion rate, which is critical during stages two and three of the diffusion welding process, and influences the metallurigical reactions which take place during the welding process such as phase transformations and grain growth. Temperatures ranging from 1500.degree. to 1850.degree. F are normally used for diffusion welding of titanium alloys. The lower temperature is approximately half the melting point of titanium, below which diffusion rates become too slow for effective bonding. Conversely, above about 1850.degree. F beta phase grain growth can become excessive in alloy titanium and will decrease the surface creep rate and necessitate the use of higher pressures to achieve surface contact.
Development of a procedure for diffusion welding a large structural section, such as a pressure hull, requires that several major engineering problems be overcome. These are:
1. Protection of the surfaces to be diffusion welded from atmospheric contamination without the use of a large vacuum chamber.
2. Application of uniform pressure to the surfaces being diffusion welded.
3. Application of the required temperature at the interface during the diffusion welding process without the use of large controlled atmosphere heat treating furnaces.
The present invention solves the above engineering problems and thus describes a unique method for the fabrication of large titanium pressure hulls.